For some examinations of the human body it is of great advantage to image only the blood vessels. One known method for this is subtraction angiography, which is based on a perfusion procedure. Basically, a first and a second image are acquired of a region of interest. Between the two images a contrast medium is introduced into the blood vessel, which absorbs X-rays. After the contrast agent is injected an X-ray imaging device records an angiographic sequence that shows the blood vessels containing the contrast agent highlighted in the X-ray image. In order to make the vessel, and especially in heart investigations the myocardial information, more accessible to the clinician, its visibility is improved. Therefore, the two images are subtracted from one another. In theory, as a result only the vascular tree filled with the contrast agent is visible. This procedure is called Digital Subtraction Angiography (DSA) in case the subtraction is done on a digital basis. DSA images are used for diagnosis and intervention purposes among others. DSA is today routinely used in vascular exams or interventions where the observed vascular structures do not move (example: in the legs, brain, etc. . . . ). But it has shown, that motion of the object between the acquisition of the first and the second image leads to disturbing artefacts in the DSA image, as background structures can only be completely eliminated where these structures are exactly aligned and have equal grey-level distributions. Its sensitivity to motion that could have occurred between the current injected frame and the corresponding mask frame (the so-called residual motion, due to heartbeat or respiration for example) is a serious disadvantage of the technique. For example, in heart investigations, patients with heavy cardiac disease ordinarily undergo cardiac catheterization. This inspection clarifies the degree of coronary stenoses and aneurism size. However, it is impossible to clarify myocardial perfusion (which is the ultimate goal of the exam) from the coronary shape. The reason is that once stenoses occur, the other normal coronaries begin to provide the ischemic muscle with blood. As a result, there is scarcely any relation between the coronary shape and myocardial perfusion. Hence, an exact image of the actual blood vessels and of the perfused regions is necessary. But the motion of the heart leads to artefacts on the DSA images.
In U.S. Pat. No. 4,729,379 the use of images corresponding to one cardiac cycle is proposed to reduce the amount of disturbing artefacts. The subtraction is performed between images of the same cardiac beat phase, thereby removing image components due to the cardiac beat from the subtraction image. In US 2007/0195932 A1 a method is described where a non-contrast region is detected in both image sequences as a reference and a mask image is selected showing the minimal positional shift in relation to the current target image. A method of eliminating motion-artefacts in X-ray imaging processes by synchronizing the radiographing of the live image, i.e. the target image, and the mask image with the heartbeat is foreseen in U.S. Pat. No. 4,903,705. The document JP 2006-051070 shows a method for improving a DSA process by automatically choosing an optimal mask image for the creation of a DSA picture, which is done by providing a so-called phase contrast evaluation function using human body analytical data for selecting a mask image that shows the smallest difference to the target image. Further, WO 03/083777 A2 describes a method where the image sequences are aligned by using reference signs, i.e. motion signals, which are examined by the means of a similarity function to determine two instants at which the object has approximately the same state of motion during the respective motions. When dealing with non-cardiac DSA (mostly neurological, anterior or posterior limb exams), residual motions mostly come from global patient motions. The resulting artefacts can be corrected to some extent by digitally compensating for the motion that has occurred between the mask image and the currently injected image. But residual motions observed in cardiac DSA appear more often, with larger amplitudes, and they are more difficult to compensate for (compared to non-cardiac DSA). This is mainly because the beating of the heart varies in pace, rhythm and amplitude, in particular when a contrast agent is injected. It is then difficult to find a matching pair of images to apply subtraction. Further, breathing could also impair the mask/injected frame matching and it is more difficult to hold its breath during an exam than to simply stay motionless. Still further, perfusion exams of the heart last longer than classical angiography exams since not only the contrast agent has to propagate in the vessels (during the arterial phase) of interest, but one has to wait for the subsequent migration of the contrast agent in the heart muscle (during the perfusion phase). This results in much longer exams (10-15 seconds vs. 2-4 seconds), which also implies more frequent and larger residual motions. Another aspect is that X-ray images are transparent. That makes the estimation and compensation of the organs that they contain particularly difficult. Indeed, if organ 1 moves over organ 2, one could either compensate for the motion of organ 1 (and thus to move artificially organ 2), or to leave the artefact created by the motion of organ 1. Non-cardiac DSA mainly needs to correct for global patient motions that do not imply any transparency effect. On the contrary, cardiac exams do involve strong transparent effects (lungs, diaphragm, heart, column and ribs can move over each other, with superimposed, possibly contradictory, vector fields). As a result, it is much more difficult to compensate for residual motions in cardiac DSA than in non-cardiac DSA. But it is crucial to limit the artefacts corrupting the subtracted images presented to the clinician as much as possible.